The present disclosure relates to rotary electric machines, and particularly to rotors therefor, and more particularly to rotor types which include permanent magnets.
An example of a prior rotary electric machine to which the teachings of the present disclosure may be applied, an alternator for use in a vehicle, is depicted in FIG. 1. Alternator 20 has a housing 24 and a rotor shaft 28 supported within the housing 24 by front and rear rolling element bearings 32 and 36, respectively. A belt driven pulley 40 is fastened to a protruding front end of the rotor shaft 28. The rotor 56 of the depicted rotary electric machine 20 includes front and rear alternator pole pieces 44 and 48, respectively, which are mounted to and rotate with the shaft 28. Alternator 20 generally includes a stator 52 which surrounds the rotor 56 and is affixed to the housing 24. Rotation of the rotor 56 about its axis of rotation, the machine central axis 60, causes an alternating current to be induced in the stator winding 68.
The stator 52 generally includes a stator core 64 about which stator winding 68 is coiled. As is known in the art, the stator core 64 generally includes a lamina stack 72 formed by a plurality of laminae stacked axially relative to the rotational axis 60 of the rotor shaft 28. Each lamina may be made of electrical steel or another suitable ferromagnetic material. Referring to FIG. 2, the stator winding 68 typically includes a plurality of conductors 76, and the stator core 64 defines a plurality of slots 80 leaving a plurality of stator teeth 84 therebetween; the stator slots 80 and teeth 84 are also shown in FIG. 10. The plurality of conductors 76 extend axially through the slots 80 and are looped in a conventional fashion such that the loops are distributed around the circumference of the stator 52. As shown in FIG. 2, the plurality of stator winding conductors 76 are namely a first conductor 76a, a second conductor 76b, and a third conductor 76c, the conductors defining three phases of electrical power generated by the alternator.
The rotor 56 is a type well-known as a claw-pole rotor, and includes the pair of opposing claw-pole pieces 44, 48 and an excitation field coil 88 disposed about the central axis 60. Pole pieces 44 and 48 are made of a magnetic material such as steel, and are substantially identical to each other, having respective hub portions 92, 96 and a plurality of respective, elongate pole segments or fingers 100, 104. The pole fingers of each pole piece 44, 48 are distributed about the circumference of the respective hub portion 92, 96 and are spaced by voids 108 in the respective hub portion. The pole fingers 100, 104 of each pole piece 44, 48 extend axially away from their respective hub portion, and axially towards the hub portion 92, 96 of the other pole piece. Further, the pole fingers 100, 104 of each pole piece 44, 48 are symmetrically spaced around the perimeter of the respective hub portion 92, 96 and, with the rotor 56 configured as assembled onto the shaft 28, are interleaved in a non-contacting, spaced relationship with the pole fingers of the other pole piece, as shown in FIG. 2. Air gaps or channels are thus defined between adjacent pole fingers 100,104 and are distributed circumferentially about the rotor 56.
Referring to FIG. 3, the excitation field coil 88 of the rotor 56 is wound upon an electrically insulative bobbin 112 and the coil 88 and bobbin 112 are sandwiched between the pair of opposing, axially inwardly facing surfaces 116, 120 of the pole piece hub portions 92, 96. Pole pieces 44, 48 may have axially-extending portions 121 about which the field coil 88 and its bobbin 112 are disposed, as shown in FIG. 1, or the field coil 88 and its bobbin 112 may be disposed about a cylindrical rotor core member 122 disposed about the central axis 60 and located between the pole pieces 44, 48, as shown in FIG. 3. Referring again to FIG. 1, DC excitation current is applied to the excitation winding 88 through a pair of slip rings 124 and associated contact brushes 128. The slip rings 124 are secured to the shaft 28 and in operation couple the field coil 88 to a regulated DC current source via the contact brushes 128. A control system known as a voltage regulator (not shown) is used to apply an appropriate level of DC voltage to the excitation windings 88.
The pole pieces 44, 48 and the energized field winding 88 produce an alternating polarity magnetic field that rotates with the rotor 56 about the central axis 60. Although a DC excitation current is applied to the field winding 88, the interlacing of the alternating pole fingers 100, 104 creates an alternating polarity magnetic flux linkage. This magnetic flux linkage is presented to the winding conductors 76 of the stationary stator 52 that surrounds the rotor 56. The movement of the alternating polarity magnetic flux linkage presented by the rotor 56 across the stator winding conductors 76a, 76b, 76c generates three-phase AC electrical power in a well-known manner.
Typically, AC electrical output by the alternator 20 is directed to a rectifier 132, which may be located at the rear of the housing 24 as shown in FIG. 1. The alternator may also include further filtering and power conditioning devices through which the electrical output is directed before it is conducted as DC electrical power to the positive terminal of the vehicle battery (not shown) or an electric distribution bus (also not shown). The desired RMS value of the outputted alternating current from the alternator 20 is dependent upon the level of DC voltage applied by the voltage regulator to the excitation windings 88. Additionally, front and rear air circulation fans 136 and 140 are located at opposite axially outward sides of the pole pieces 44, 48. The fans 136, 140 are coupled to the rotor 56 and rotate in unison therewith. Cooling airflow is typically drawn axially inwardly of the housing 24, and is expelled radially outwardly of the housing 24, by the fans 136, 140. The rear fan 140 typically directs cooling airflow across the rectifier 132 and other electronic components of the alternator 20. If an airflow path is provided, the fans 136, 140 may also direct some amount of cooling airflow around the pole fingers 100, 104 and the excitation coil 88.
The direction of rotation of the rotor 56 relative to the stator 52, and thus the direction of movement of the rotor pole fingers 100, 104 relative to the stator teeth 84 is shown by arrow 144. Upon energization of the field coil 88 with a regulated DC current the rotor 56 is magnetized, with the adjacent pole fingers 100, 104 alternating circumferentially between north (N) and south (S) magnetic polarities. In other words, all pole fingers 100 have N magnetic polarity and all pole fingers 104 have S magnetic polarity. Accordingly, it will be recognized that upon rotation of the rotor 56, the alternating magnetic polarities of the pole fingers 100, 104 pass sequentially around the stator 52, thereby inducing an output current in the stator winding 68. Those of ordinary skill in the art will recognize that the respective N and S magnetic polarities of the front and rear pole pieces 44, 48 are determined as a function of the chosen direction of DC current flow through the excitation field coil 88.
FIGS. 5A-5H show an example of a prior claw-pole piece 44 or 48 including a plurality of pole fingers or segments 100, 104 each having a base or proximal end 148 connected to the respective pole piece hub portion 92, 96 at locations between the voids 108. Each pole finger 100, 104 also has a tip or distal end 152 opposite is respective base 148, and the tips 152 of the pole fingers 100, 104 of one pole piece 44, 48 are located near the base 148 of the pole fingers 100, 104 of the other pole piece 44, 48, as shown in FIG. 3.
Each pole finger 100, 104 also has a leading edge 156 and an opposite trailing edge 160, each of which extends between the base 148 and the tip 152 of the pole finger. The designation of an edge 156, 160 as leading or trailing is related to the direction of pole finger travel relative to the stator core teeth 84, as indicated by arrow 144. The leading and trailing edges 156, 160 of each pole finger 100 of front pole piece 44 respectively define leading edge side surface 164 and trailing edge side surface 168; the leading and trailing edges 156, 160 of each pole finger 104 of rear pole piece 48 respectively define leading edge side surface 172 and trailing edge side surface 176.
Each pole finger 100 also defines a radially outer surface 180 and a radially inner surface 184, each of which extends circumferentially between its opposite leading and trailing edge side surfaces 164, 168. Each pole finger 104 also defines a radially outer surface 188 and a radially inner surface 192, each of which extends circumferentially between its opposite leading and trailing edge side surfaces 172, 176. As shown in FIGS. 2 and 5A, each radially outer surface 180, 188 lies along a respective surface line 196 that is substantially parallel with central axis 60, such that a cylinder may be defined by the arranged plurality of surface lines 196. Thus, the radially outer surfaces 180, 188 of the plurality of alternating pole fingers 100, 104 define the substantially cylindrical outer circumferential surface of the rotor 56.
Relative to each pole finger 100, 104 shown in FIGS. 1-8 and 10, which depict them as having a generally pyramidal shape, the respective radially inner surface 184, 192 is closer to the central axis 60 near its base or proximal end 148, and further from the central axis 60 near its tip or distal end 152, which may be flattened, as shown. Thus, each pyramidal pole finger 100, 104 is thicker radially, relative to the axis 60, between its radially outer surface 180, 188 and its radially inner surface 184, 192, at its proximal end or base 148 than at its distal end or tip 152. Additionally, when viewed in a radial direction each pyramidal pole finger 100, 104 is tapered as the pole finger extends away from its respective hub portion 92, 96 and therefore is circumferentially wider between its leading and trailing edges 156, 160 at its proximal end 148 and narrower at its distal end 152. It can therefore be understood that each pole finger 100, 104 may be generally V-shaped as viewed in both a radial direction relative to the central axis 60, and in a direction normal to an imaginary plane in which the respective surface line 196 and the central axis 60 both lie. In other words, each generally pyramidal pole finger 100, 104, if sectioned at its base 148 by an imaginary plane oriented perpendicular to the central axis 60 and flattened at its tip, is substantially hexahedral.
Moreover, as can be clearly understood from the various views of FIGS. 1-8, in imaginary planes perpendicular to the central axis 60, at varying distances axially along each pyramidal pole finger 100, 104 (that is, at various axial locations in directions generally parallel with the central axis 60, the respective thickness of each pole finger between its radially outer surface 180, 188 and its radially inner surface 184, 192 is substantially uniform between its leading and tailing edges 156, 160. Additionally, but for radially inner and outer surfaces 180, 188, 184, and 192 presenting slight curvatures about the central axis 60 corresponding to the cylindrical shape of the rotor 56 (convex in the case of radially outer surface 180, 188, and concave in the case of radially inner surface 184, 192), these surfaces 180, 188, 184, and 192 are generally flat and featureless between their respective pole finger leading and trailing edges 156, 160.
In some prior machines 20 the pole fingers 100, 104, rather than being generally pyramidal in shape as discussed above, instead have a different geometry. For example, referring to FIG. 9, the pole pieces 44, 48 may instead define pole fingers or segments 100, 104 that are generally rectangular in shape when viewed radially relative to the central axis 60. As in the case of the generally pyramidal pole segments described above, the pole fingers or segments 100, 104 of the prior claw-pole pieces 44 or 48 shown in FIG. 9 each have: a base or proximal end 148 connected to the respective pole piece hub portion 92, 96 at locations between the voids 108; a tip or distal end 152 opposite its respective base 148, with the tip 152 of the pole finger of one pole piece 44, 48 being located near the base 148 of the pole finger of the other pole piece 44, 48; a leading edge 156; and an opposite trailing edge 160, the leading and trailing edges 156, 160 extending between the pole finger base 148 and tip 152. The generally parallel leading and trailing edges 156, 160 of each pole finger 100 respectively define the leading edge side surface 164 and the trailing edge side surface 168, whereas the leading and trailing edges 156, 160 of each pole finger 104 respectively define the leading edge side surface 172 and trailing edge side surface 176. As discussed above, the designation of an edge 156, 160 as leading or trailing is related to the direction of pole finger travel relative to the stator core teeth 84, as indicated by arrow 144.
Unlike the generally pyramidal pole segments described above, however, in the example of FIG. 9 the leading and trailing edges 156, 160 are generally parallel to each other and to the central axis 60. Here, the depicted pole finger tips or distal ends 152 are flat, and each pole finger 100, 104, if sectioned at its base 148 by an imaginary plane oriented perpendicular to the central axis 60, may be substantially hexahedral. In the example depicted in FIG. 9, each pole finger 100, 104 respectively defines a radially outer surface 180, 188 and a radially inner surface 184, 192 (not shown in FIG. 9). As in the case of the generally pyramidal pole fingers, each radially outer, rectangular surface 180, 188 lies along a respective surface line 196 that is substantially parallel with the central axis 60, whereby the cylindrical rotor shape may be defined by the arranged plurality of surface lines 196. Relative to each pole finger 100, 104, its respective radially outer surface 180, 188 extends a substantially uniform distance between the circumferentially opposite leading edge 156 and trailing edge 160; similarly, its respective radially inner surface 184, 192 extends a substantially uniform distance between the circumferentially opposite leading edge 156 and trailing edge 160. Thus, each pole finger 100, 104 has a generally rectangular shape when viewed in a radial direction, as mentioned above.
Furthermore the pole fingers 100, 104 depicted in FIG. 9 may each be substantially configured as a rectangular parallelepiped or cuboid, wherein, as viewed in a direction normal to an imaginary plane in which the respective surface line 196 and the central axis 60 both lie, the thickness of each pole finger 100, 104 between its respective radially outer surface 180, 188 and radially inner surface 184, 192, is substantially uniform along its axial direction, i.e., in a direction generally parallel with the surface line 196. Thus, each pole finger 100, 104 has a generally rectangular shape when viewed in a tangential direction, perpendicular to the central axis 60. Furthermore, but for surfaces 180, 188, 184, 192 presenting slight curvatures about the central axis 60 corresponding to the cylindrical shape of the rotor 56 (convex in the case of radially outer surface 180, 188, and concave in the case of radially inner surface 184, 192), the surfaces 180, 188, 184, and 192 of the generally cuboid pole fingers 100, 104 are generally flat between their respective pole finger leading and trailing edges 156, 160. Moreover, the opposed radially outer surface 180, 188 and radially inner surface 184, 192 of each generally cuboid pole finger 100, 104 are substantially parallel. In other words, in imaginary planes perpendicular to the central axis 60, at varying distances axially along each pyramidal pole finger 100, 104 (that is, at distances in directions generally parallel with surface lines 196), the respective thickness of each pole finger between its radially outer surface 180, 188 and radially inner surface 184, 192 is substantially uniform. A prior electrical machine including pole fingers or pole segments having leading and trailing edges substantially parallel with each other and the machine central axis is also disclosed in U.S. Pat. No. 7,973,444 entitled ELECTRIC MACHINE AND ROTOR FOR THE SAME and assigned to the assignee of the present application, the entire disclosure of which is expressly incorporated herein by reference.
As noted above, regardless of whether their pole fingers 100, 104 are generally pyramidal or generally cuboid, in prior rotary electrical machines such as an alternator 20 the pole finger radially inner surfaces 184, 192 are substantially flat or provided with only a very minor concave curvature about the central axis 60 between their respective leading and trailing edges 156, 160, at various locations along the axial length of the pole finger, i.e., in directions parallel with surface lines 196. The curvature of the radially inner surface 184, 192, where present, is more pronounced near the pole finger base or proximal end 148 than it is near the pole finger tip or distal end 152, as revealed by comparisons between FIGS. 5F-5H, and between FIGS. 7B-7E.
It is also known to employ permanent magnets in the rotors of rotary electrical machines such as alternators. In some prior alternators, high-magnetic-strength permanent magnets 200 are disposed between the adjacent claw-pole fingers 100, 104 to supplement the magnetic field generated by the excitation coil 88. Such magnets 200, which are optional, are shown in FIGS. 6-9. Any of a variety of permanent magnet material may be used for permanent magnets 200 such as neodymium-iron-boron, samarium-cobalt, or ferrite. Alternators utilizing both field coil and permanent magnet fluxes coupled to a stator coil are referred to as hybrid alternators. Referring to FIG. 10, in a hybrid alternator 20, permanent magnets 200 maintain a permanent magnet flux across channels 204 that would otherwise be air gaps between the claw-pole segments 100, 104, which in a hybrid alternator are magnetically linked to the permanent magnets 200 disposed in channels 204 and carried by the rotor 56, and a portion of the stator structure 52, thereby coupling significant magnetic flux through the stator structure. The magnetic flux path 208 is shown in dashed lines in FIG. 10. When the field coil 88 is not energized, the magnetic flux developed by the permanent magnets 200 is shunted through the rotor 56. However, when the field coil 88 is energized, the magnetic flux developed by the permanent magnets 200 additively contributes to the electromagnetically generated magnetic flux resulting from field coil excitation, across the stator/rotor air gap 212. Depending on the desired output of the hybrid alternator 20, the effect of the permanent magnets 200 on the flux across the radial stator/rotor air gap 212 may supplement, or boost, the electromagnetic flux generated by the DC current being passed in one direction through the field effect coil 88; the effect of the permanent magnets on the flux across the stator/rotor air gap may also be reduced, or bucked, by electromagnetic flux that is generated by DC current being passed in the opposite direction through the field effect coil 88. Alternator buck/boost control circuits are known in the art and may be of various designs, one of which is disclosed in above-mentioned U.S. Pat. No. 7,973,444.
Channels 204 may be oriented as described above; typically, the orientation and shape of the permanent magnets 200 is similar. Thus, permanent magnets 200 are generally prism-shaped with six substantially flat faces. The permanent magnets 200 being substantially prism-shaped provides substantially symmetrical abutting surfaces at their respective interfaces with the leading and trailing edge side surfaces 164, 168, 172, 176. The prism-shaped permanent magnets 200 are illustrated herein as an exemplary shape, it being understood that other shapes for the permanent magnets will be apparent to the skilled artisan. As shown herein, each permanent magnet 200 has a pair of circumferentially opposing pole faces 216, with the polarized faces 216N and 216S corresponding to N and S magnetic polarities, respectively. The polarities of the permanent magnets alternate such that adjacent magnets are of opposite polarity. Therefore, it can be appreciated that claw-pole fingers 100 abut permanent magnet pole faces 216N and have a first common polarity (i.e., N), and claw-pole fingers 104 abut permanent magnet pole faces 216S and have a second common polarity (i.e., S). The pole faces 216N, 216S of magnets 200 are immediately adjacent respective leading and trailing edge side surfaces 164, 168, 172, 176 on pole fingers 100 and 104. As mentioned above, all pole fingers 100 have N magnetic polarity and all pole fingers 104 have S magnetic polarity. All permanent magnet pole faces 216N are adjacent the side surfaces 164, 168 of each N pole finger 100. Likewise, all permanent magnet pole faces 216S are adjacent the side surfaces 172, 176 of each S pole finger 104. The foregoing arrangement is generally well known to those skilled in the art.
Typically, when permanent magnets 200 are added between the claw poles 100, 104 of an alternator 20 to boost machine performance, the air gap channel 204 is machined or otherwise adapted to provide a constant width between the opposing pole finger leading and trailing edge side surfaces 164, 168, 172, 176 to contain the magnets. However, the shape of the claw-pole pieces 44, 48 used in prior hybrid alternators is not optimized to maximize the use of the permanent magnets 200. Rather, the pole piece designs of such hybrid machines, and particularly the designs of their pole segments or fingers 100, 104 are “carried over” from a conventional, non-permanent magnet-equipped claw-pole rotor design, which had already evolved to maximize machine performance without the addition of permanent magnets to the claw-pole rotor. To simply adhere to this practice does not take full advantage of the benefit of adding magnets to claw-pole rotors.
A rotary electric machine configured to maximize the beneficial aspects of a permanent magnet-equipped rotor would provide a desirable improvement in the art.